This invention is directed generally to the art of self-unloading marine vessels and, more particularly, to self-unloading barges that contain hoppers for discharging bulk cargo. Examples of bulk cargo include stone, sand, coal, and taconite.
Conventional self-unloading bulk-cargo barges that operate, for example, in the Great Lakes have a plurality of hoppers that are loaded from above with the bulk material to be transported. The hoppers are arranged in one or more rows parallel to the keel of the barge, and a conveyor system is located beneath each row of hoppers in the hold of the barge. At the bottom of each hopper is a cargo discharge gate designed to prevent passage of the cargo out of the bottom of the hopper until the barge reaches its destination and unloading begins.
When the barge is to be unloaded at its destination, the cargo discharge gates are opened, permitting the bulk cargo to be discharged by gravity from the hoppers onto the conveyor system below. The conveyor system in the hold of the barge typically transports the cargo being discharged from the hoppers to an above-deck unloading station via a series of ramps.
Conventional cargo discharge gates are not capable of preventing water that down-floods into a hopper from flowing out of the bottom of the hopper. Thus, some means must be employed to prevent heavy seas and rainfall from entering the hoppers, passing through the cargo discharge gates, and entering the void spaces and machinery spaces of the barge, where the water possibly could flood compartments and de-stabilize the barge. To protect against such water damage, bulk-cargo barges conventionally are fitted with hatch covers over the hoppers. Typically, each of these hatch covers spans a plurality of hoppers. Each hatch cover is required to provide a weather-tight seal along its peripheral edge to prevent water that splashes on the barge's deck during inclement weather and/or high seas from entering the hoppers.
Although heretofore required for safe operation and compliance with shipping regulations, these hatch covers interfere with the efficient operation of bulk-cargo barges in significant ways. If the necessity for hatch covers could be eliminated, the amount of cargo transported by a barge during each trip could be increased, and the costs of operating such barges could be substantially reduced.
First, the use of hatch covers imposes a weight penalty. Hatch covers are extremely heavy, in part because they must be structurally sound to withstand the stress resulting from water splashing on them during rough sees. In addition, the use of hatch covers usually requires that the barge carry a handling device dedicated to removing and replacing the covers. Typically, a bridge crane operating on rails is used to retract and extend the hatch covers. This crane and its associated support structure also are heavy. The total weight of the hatch covers and associated coamings, decking, stiffeners, crane, and crane rails easily can reach 300-400 short tons. The cargo-carrying capacity of the barge must be reduced by an equal amount.
Moreover, the added weight of the hatch covers and associated structure is concentrated above the center of gravity of the vessel, which also interferes with efficient loading. The weight of these structures increases the height of the vessel's center of gravity and consequently increases the difference between the vessel's center of gravity and its center of buoyancy. When this factor is taken into account in stability calculations, the maximum payload of the vessel must be decreased further.
Hatch covers also limit the volume of cargo that can be loaded into a barge. Particularly for bulk cargo having a relatively low mass density, such as coal, loading must be stopped long before the vessel's maximum load limit is reached to permit the fitment of the hatch covers over the hoppers. If the necessity of hatch covers were eliminated, a barge more often could be loaded to its maximum weight capacity.
Hatch covers also are expensive to purchase as original equipment and are costly to maintain in the requisite weather-tight condition. For one typical barge application having two rows of hoppers, the aggregate area of the hopper tops totals 13,400 square feet. Hatch covers for this barge would require at least that much in sheet steel and would require maintaining a weather-tight seal along a perimeter of about 1500 feet.
In addition, hatch covers significantly add to the operating cost of a bulk-cargo vessel. In addition to the cost of maintaining the crane required for moving the hatch covers, utilization of hatch covers interferes with efficient loading of the vessel. Because covers must be fit over discrete areas of the barge, the loading operation must make a stop when the capacity of each hatch opening is reached. Continuous loading of a bulk-cargo barge without concern for the fitment of hatch covers would take 40-50% less time than the discontinuous loading of a conventional barge.
The present invention is intended to enable safe operation of a bulk-cargo vessel without the use of hatch covers.
The present invention is intended to provide a water-tight seal at the bottom of each hopper of a bulk-cargo vessel, thus enabling the elimination of hatch covers.
Furthermore, the present invention is intended to provide a bulk-cargo vessel with weirs for limiting the amount of water that accumulates in hoppers having water-tight seals at their bottoms when the vessel is operated without hatch covers.
By eliminating the necessity for hatch covers, the present invention offers significant advantages over conventional bulk-cargo vessels. First, the substantial cost of hatch covers and the requisite equipment for moving them is eliminated, reducing the cost of the vessel. Second, eliminating the hatch covers and associated structure and equipment provides for increasing the vessel's load capacity in most instances. Third, the operating costs for loading the vessel are reduced, because the vessel can be loaded on a continuous basis.
Additional advantages of the present invention will be set forth in part in the description that follows, and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the apparatus particularly pointed out in the appended claims.